CI: one test binary per crate, nextest, line tables only (#3855)

* CI: line tables only for workspace crates, no debug info for dependencies

* CI: one integration-test binary per crate

Each crate's tests/*.rs files become modules of tests/it/main.rs. Files that change
process-wide state (Color Settings, the font database, installed plug-ins, the working
directory), the fuzz sweeps and the test targets the Affinity corpus workflow names stay
their own binaries.

* Docs: name the integration tests' new paths

* CI: xtask test with nextest and doctests, one wasm check, clippy --keep-going

cargo xtask ci runs the tests with cargo nextest (cargo test when it is not installed),
builds them with --lib --bins --tests, runs the doctests as their own step, and checks
the wasm crates in one cargo check --keep-going call.

crates/affinity/src/tests.rs

@@ -1,4 +1,4 @@
-//! Unit tests on synthetic inputs; `tests/real_files.rs` covers public Affinity documents.
+//! Unit tests on synthetic inputs; `tests/it/real_files.rs` covers public Affinity documents.
 
 mod preview {
     use crate::container::header;